Aqueous polymerization of fluorinated monomer using polymerization agent comprising fluoropolyether acid or salt and short chain fluorosurfactant

ABSTRACT

A process includes polymerizing at least one fluorinated monomer in an aqueous medium containing initiator and polymerization agent to form an aqueous dispersion of particles of fluoropolymer. The polymerization agent includes fluoropolyether acid or salt having an M n  of at least about 800 g/mol and fluorosurfactant having the formula: [R 1 —O n -L-A − ] Y + . R 1  is a linear or branched partially or fully fluorinated aliphatic group which may contain ether linkages; n is 1; L is a linear or branched alkylene group which may be nonfluorinated, partially fluorinated, or fully fluorinated and may contain ether linkages; A −  is a carboxylate; and Y +  is hydrogen, ammonium, or alkali metal cation. The chain length of R 1 —O n -L- is not greater than 6 atoms. The polymerization agent includes a minor amount of fluoropolyether acid or salt thereof and a major amount of fluorosurfactant. The polymerizing produces less than about 3 wt % undispersed fluoropolymer based on the total weight of fluoropolymer produced.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 13/965,317, filed Aug. 13, 2013, which is a continuation ofU.S. patent application Ser. No. 13/313,271, filed Dec. 7, 2011, whichis abandoned, which is a continuation of U.S. patent application Ser.No. 12/712,278, filed Feb. 25, 2010, which is abandoned, which is adivision of U.S. patent application Ser. No. 11/937,544, filed Nov. 9,2007, which issued as U.S. Pat. No. 7,705,074 on Apr. 27, 2010, whichclaims the benefit of U.S. Provisional App. No. 60/857,960 and U.S.Provisional App. No. 60/858,010, both filed Nov. 9, 2006.

FIELD OF THE INVENTION

This invention relates to a process for the dispersion polymerization offluorinated monomer in an aqueous polymerization medium.

BACKGROUND OF THE INVENTION

A typical process for the aqueous dispersion polymerization offluorinated monomer includes feeding fluorinated monomer to a heatedreactor containing a fluorosurfactant and deionized water. Paraffin waxis employed in the reactor as a stabilizer for some polymerizations,e.g., polytetrafluoroethylene (PTFE) homopolymers. A free-radicalinitiator solution is employed and, as the polymerization proceeds,additional fluorinated monomer is added to maintain the pressure. Achain transfer agent is employed in the polymerization of some polymers,e.g., melt-processible TFE copolymers to control melt viscosity. Afterseveral hours, the feeds are stopped, the reactor is vented and purgedwith nitrogen, and the raw dispersion in the vessel is transferred to acooling vessel.

For use in fluoropolymer coatings for metals, glass and fabric, polymerdispersion is typically transferred to a dispersion concentrationoperation which produces stabilized dispersions used as coatings.Certain grades of PTFE dispersion are made for the production of finepowder. For this use, the dispersion is coagulated, the aqueous mediumis removed and the PTFE is dried to produce fine powder. Dispersions ofmelt-processible fluoropolymers for molding resin use are alsocoagulated and the coagulated polymer dried and then processed into aconvenient form such as flake, chip or pellet for use in subsequentmelt-processing operations.

As described in U.S. Pat. No. 3,391,099 to Punderson, dispersionpolymerization involves two generally distinct phases. The initialperiod of the reaction is a nucleation phase in which a given number ofpolymerization sites or nuclei are established. Subsequently, thereoccurs a growth phase in which polymerization of fluorinated monomer onestablished particles occurs with little or no formation of newparticles. Successful production of the high solids fluoropolymerdispersion generally requires the presence of the fluorosurfactant,especially in the later growth phase of polymerization in order tostabilize the dispersion preventing coagulation of the fluoropolymerparticles.

Fluorosurfactants used in the polymerization are usually anionic,non-telogenic, soluble in water and stable to reaction conditions. Themost widely used fluorosurfactants are perfluoroalkane carboxylic acidsand salts as disclosed in U.S. Pat. No. 2,559,752 to Berry, specificallyperfluorooctanoic acid and salts, often referred to as C8, andperfluorononanoic acid and salts, often referred to as C9. Because ofrecent environmental concerns with regard to perfluorooctanoic acid andsalts, there is interest in reducing or eliminating perfluorooctanoicacid and its salts in fluoropolymer polymerization processes.

There has been a similar environmental concern with regard toperfluorooctane sulfonate (PFOS), an 8 carbon fluorosurfactant formerlysold by 3M as a stain repellent under the trademark Scotchguard®. Foruses such as stain repellency, fluorosurfactants with short hydrophobicchain lengths, e.g., perfluorobutane sulfonate, have been used toreplace perfluorooctane sulfonate. However, if attempts are made toemploy short hydrophobic chain length fluorosurfactants in thecommercial polymerization of fluoromonomers to achieve desirable solidsconcentrations, significant amounts of undispersed polymer (alsoreferred to as coagulum) will form in the reactor. This coagulumtypically must be discarded as waste. For example, the short chainfluoroether CF₃CF₂CF₂OCF(CF₃)COOH is disclosed in Example XII of U.S.Pat. No. 3,271,341 to Garrison as a polymerization surfactant forpolytetrafluoroethylene (PTFE). However, in this example which produces1243 grams of PTFE dispersion, 500 grams of coagulum is produced, i.e.,29% of the total weight of PTFE produced is coagulum.

Rather than employ short chain fluorosurfactants, other known methodsemploy higher molecular weight materials such as fluoropolyethers in thepolymerization of fluoropolymers. U.S. Pat. No. 4,864,006 to Gianetti etal. discloses the polymerization of fluorinated monomers in the presenceof a perfluoropolyether having neutral end groups, perfluoropolyetheroil, which is used in the form of an aqueous microemulsion. Theperfluoropolyether oil has molecular weight of at least about 500 andthe aqueous microemulsion of the oil is prepared using a suitablesurfactant which can be selected from known perfluorinated carboxylic orsulfonic acids or from perfluoropolyethers having one or two acid endgroups. U.S. Pat. No. 6,395,848 to Morgan et al. discloses an improvedprocess for the aqueous dispersion polymerization of fluorinatedmonomers using a combination of fluorosurfactant, which can be afluoroalkyl carboxylic or sulfonic acid or salt thereof or fluoroalkoxyaryl sulfonic acid or salt thereof, and perfluoropolyether carboxylic orsulfonic acid or salt thereof. The perfluoropolyether carboxylic orsulfonic acid or salt thereof employed in the examples of Morgan et al.have molecular weights ranging from 2000 to 7500. The fluoroalkylcarboxylic or sulfonic acid surfactants disclosed in Morgan et al. arethose commonly used in dispersion polymerization such asperfluorooctanoic acid or salts and other fluorosurfactants with thesame or longer chain length which can be used alone as polymerizationsurfactants.

SUMMARY OF THE INVENTION

The invention is based on the discovery that a fluoropolyether acid orsalt having a number average molecular weight of at least about 800g/mol in combination with a fluorosurfactant having a chain length of nogreater than 6 provides an effective polymerization agent for use in themanufacture of fluoropolymers. The process of the invention comprisespolymerizing at least one fluorinated monomer in an aqueous mediumcontaining initiator and polymerization agent to form an aqueousdispersion of particles of fluoropolymer, said polymerization agentcomprising:

fluoropolyether acid or salt thereof having a number average molecularweight of at least about 800 g/mol; and

fluorosurfactant having the formula:

[R¹—O_(n)-L-A⁻]Y⁺

wherein:

-   -   R¹ is a linear or branched partially or fully fluorinated        aliphatic group which may contain ether linkages;    -   n is 1;    -   L is a linear or branched alkylene group which may be        nonfluorinated, partially fluorinated or fully fluorinated and        which may contain ether linkages;    -   A⁻ is a carboxylate; and    -   Y⁺ is hydrogen, ammonium or alkali metal cation;    -   with the proviso that the chain length of R¹—O_(n)-L- is not        greater than 6 atoms,

wherein said polymerization agent comprises a minor amount of saidfluoropolyether acid or salt thereof and a major amount of saidfluorosurfactant; and

wherein said polymerizing produces less than about 3 wt % undispersedfluoropolymer based on the total weight of fluoropolymer produced.

DETAILED DESCRIPTION OF THE INVENTION Fluoropolymer

Fluoropolymer dispersions formed by this invention are comprised ofparticles of fluoropolymer made from at least one fluorinated monomer,i.e., wherein at least one of the monomers contains fluorine, preferablyan olefinic monomer with at least one fluorine or a perfluoroalkyl groupattached to a doubly-bonded carbon. The fluorinated monomer used in theprocess of this invention is preferably independently selected from thegroup consisting of tetrafluoroethylene (TFE), hexafluoropropylene(HFP), chlorotrifluoroethylene (CTFE), trifluoroethylene,hexafluoroisobutylene, perfluoroalkyl ethylene, fluorovinyl ethers,vinyl fluoride (VF), vinylidene fluoride (VF2),perfluoro-2,2-dimethyl-1,3-dioxole (PDD),perfluoro-2-methylene-4-methyl-1,3-dioxolane (PMD), perfluoro(allylvinyl ether) and perfluoro(butenyl vinyl ether). A preferredperfluoroalkyl ethylene monomer is perfluorobutyl ethylene (PFBE).Preferred fluorovinyl ethers include perfluoro(alkyl vinyl ether)monomers (PAVE) such as perfluoro(propyl vinyl ether) (PPVE),perfluoro(ethyl vinyl ether) (PEVE), and perfluoro(methyl vinyl ether)(PMVE). Non-fluorinated olefinic comonomers such as ethylene andpropylene can be copolymerized with fluorinated monomers.

Fluorovinyl ethers also include those useful for introducingfunctionality into fluoropolymers. These includeCF₂═CF—(O—CF₂CFR_(f))_(a)—O—CF₂CFR′_(f)SO₂F, wherein R_(f) and R′_(f)are independently selected from F, Cl or a perfluorinated alkyl grouphaving 1 to 10 carbon atoms, a=0, 1 or 2. Polymers of this type aredisclosed in U.S. Pat. No. 3,282,875 (CF₂═CF—O—CF₂CF(CF₃)—O—CF₂CF₂SO₂F,perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride)), and in U.S.Pat. Nos. 4,358,545 and 4,940,525 (CF₂═CF—O—CF₂CF₂SO₂F). Another exampleis CF₂═CF—O—CF₂— CF(CF₃)—O—CF₂CF₂CO2CH₃, methyl ester ofperfluoro(4,7-dioxa-5-methyl-8-nonenecarboxylic acid), disclosed in U.S.Pat. No. 4,552,631. Similar fluorovinyl ethers with functionality ofnitrile, cyanate, carbamate, and phosphonic acid are disclosed in U.S.Pat. Nos. 5,637,748; 6,300,445; and 6,177,196.

The invention is especially useful when producing dispersions ofpolytetrafluoroethylene (PTFE) including modified PTFE. PTFE andmodified PTFE typically have a melt creep viscosity of at least about1×10⁸ Pa·s and, with such high melt viscosity, the polymer does not flowsignificantly in the molten state and therefore is not amelt-processible polymer. Polytetrafluoroethylene (PTFE) refers to thepolymerized tetrafluoroethylene by itself without any significantcomonomer present.

Modified PTFE refers to copolymers of TFE with such small concentrationsof comonomer that the melting point of the resultant polymer is notsubstantially reduced below that of PTFE. The concentration of suchcomonomer is preferably less than 1 wt %, more preferably less than 0.5wt %. A minimum amount of at least about 0.05 wt % is preferably used tohave significant effect. The modified PTFE contains a small amount ofcomonomer modifier which improves film forming capability during baking(fusing), such as perfluoroolefin, notably hexafluoropropylene (HFP) orperfluoro(alkyl vinyl ether) (PAVE), where the alkyl group contains 1 to5 carbon atoms, with perfluoro(ethyl vinyl ether) (PEVE) andperfluoro(propyl vinyl ether) (PPVE) being preferred.Chlorotrifluoroethylene (CTFE), perfluorobutyl ethylene (PFBE), or othermonomer that introduces bulky side groups into the molecule are alsoincluded.

The invention is especially useful when producing dispersions ofmelt-processible fluoropolymers. By melt-processible, it is meant thatthe polymer can be processed in the molten state (i.e., fabricated fromthe melt into shaped articles such as films, fibers, and tubes etc. thatexhibit sufficient strength and toughness to be useful for theirintended purpose) using conventional processing equipment such asextruders and injection molding machines. Examples of suchmelt-processible fluoropolymers include homopolymers such aspolychlorotrifluoroethylene or copolymers of tetrafluoroethylene (TFE)and at least one fluorinated copolymerizable monomer (comonomer) presentin the polymer usually in sufficient amount to reduce the melting pointof the copolymer substantially below that of TFE homopolymer,polytetrafluoroethylene (PTFE), e.g., to a melting temperature nogreater than 315° C.

A melt-processible TFE copolymer typically incorporates an amount ofcomonomer into the copolymer in order to provide a copolymer which has amelt flow rate (MFR) of about 1-100 g/10 min as measured according toASTM D-1238 at the temperature which is standard for the specificcopolymer. Preferably, the melt viscosity is at least about 10² Pa·s,more preferably, will range from about 10² Pa·s to about 10⁶ Pa·s, mostpreferably about 103 to about 105 Pa·s measured at 372° C. by the methodof ASTM D-1238 modified as described in U.S. Pat. No. 4,380,618.Additional melt-processible fluoropolymers are the copolymers ofethylene (E) or propylene (P) with TFE or CTFE, notably ETFE, ECTFE andPCTFE.

A preferred melt-processible copolymer for use in the practice of thepresent invention comprises at least about 40-98 mol %tetrafluoroethylene units and about 2-60 mol % of at least one othermonomer. Preferred comonomers with TFE are perfluoroolefin having 3 to 8carbon atoms, such as hexafluoropropylene (HFP), and/or perfluoro(alkylvinyl ether) (PAVE) in which the linear or branched alkyl group contains1 to 5 carbon atoms. Preferred PAVE monomers are those in which thealkyl group contains 1, 2, 3 or 4 carbon atoms, and the copolymer can bemade using several PAVE monomers. Preferred TFE copolymers include FEP(TFE/HFP copolymer), PFA (TFE/PAVE copolymer), TFE/HFP/PAVE wherein PAVEis PEVE and/or PPVE, MFA (TFE/PMVE/PAVE wherein the alkyl group of PAVEhas at least two carbon atoms) and THV (TFE/HFP/VF2).

Further useful polymers are film forming polymers of polyvinylidenefluoride (PVDF) and copolymers of vinylidene fluoride as well aspolyvinyl fluoride (PVF) and copolymers of vinyl fluoride.

The invention is also useful when producing dispersions of fluorocarbonelastomers. These elastomers typically have a glass transitiontemperature below 25° C. and exhibit little or no crystallinity at roomtemperature. Fluorocarbon elastomer copolymers made by the process ofthis invention typically contain 25 to 70 wt %, based on total weight ofthe fluorocarbon elastomer, of copolymerized units of a firstfluorinated monomer which may be vinylidene fluoride (VF2) ortetrafluoroethylene (TFE). The remaining units in the fluorocarbonelastomers are comprised of one or more additional copolymerizedmonomers, different from said first monomer, selected from the groupconsisting of fluorinated monomers, hydrocarbon olefins and mixturesthereof. Fluorocarbon elastomers prepared by the process of the presentinvention may also, optionally, comprise units of one or more cure sitemonomers. When present, copolymerized cure site monomers are typicallyat a level of 0.05 to 7 wt %, based on total weight of fluorocarbonelastomer. Examples of suitable cure site monomers include: i) bromine-,iodine-, or chlorine-containing fluorinated olefins or fluorinated vinylethers; ii) nitrile group-containing fluorinated olefins or fluorinatedvinyl ethers; iii) perfluoro(2-phenoxypropyl vinyl ether); and iv)non-conjugated dienes.

Preferred TFE based fluorocarbon elastomer copolymers include TFE/PMVE,TFE/PMVE/E, TFE/P and TFE/P/VF2. Preferred VF2 based fluorocarbonelastomer copolymers include VF2/HFP, VF2/HFP/TFE, and VF2/PMVE/TFE. Anyof these elastomer copolymers may further comprise units of cure sitemonomer.

Fluorosurfactant

The fluorosurfactant employed in accordance with the invention isfluorosurfactant having the formula:

[R¹—O_(n)-L-A⁻]Y⁺  (1)

wherein:

-   -   R¹ is a linear or branched partially or fully fluorinated        aliphatic group which may contain ether linkages;    -   n is 0 or 1;    -   L is a linear or branched alkylene group which may be        nonfluorinated, partially fluorinated or fully fluorinated and        which may contain ether linkages;    -   A⁻ is an anionic group selected from the group consisting of        carboxylate, sulfonate, sulfonamide anion, and phosphonate; and    -   Y⁺ is hydrogen, ammonium or alkali metal cation, with the        proviso that the chain length of R¹—O_(n)-L- is not greater than        6 atoms.

“Chain length” as used in this application refers to the number of atomsin the longest linear chain in the hydrophobic tail of thefluorosurfactant employed in the process of this invention. Chain lengthincludes atoms such as oxygen atoms in addition to carbon in the chainof hydrophobic tail of the surfactant but does not include branches offof the longest linear chain or include atoms of the anionic group, e.g.,does not include the carbon in carboxylate. “Short chain” as used inthis application refers to a chain length of not greater than 6. “Longchain” refers to a chain length of greater than 6, e.g.,fluorosurfactants having a chain length of 7 to 14 atoms.

Preferably, the chain length of R¹—O_(n)-L- is 3 to 6 atoms. Inaccordance with one preferred form of the invention the chain length ofR¹—O_(n)-L- is 4 to 6 atoms. In accordance with another preferred formof the invention the chain length of R¹—O_(n)-L- is 3 to 5 atoms. Mostpreferably, the chain length of R¹—O_(n)-L- is 4 to 5 atoms.

Typically, the surfactants employed in accordance with the inventionhave a surface tension values significantly higher than the surfacetension values of perfluorooctanoic acid and salts under the sameconditions. A polymerization agent comprising fluoropolyether acid orsalt thereof having a number average molecular weight of at least about800 g/mol in combination with the fluorosurfactant employed inaccordance with the invention has a surface tension significantly lowerthat the surfactant used alone. This is illustrated in Table A of theSurface Tension Examples which follow. This effect is surprising basedon the disclosure of U.S. Pat. No. 6,395,848 to Morgan et al. Morgan etal. discloses that the use of a perfluoropolyether having carboxylicacid ends has little if any effect on surface tension in combinationwith the surfactant 6,2-TBS, a surfactant with a chain length of 8atoms.

In accordance with a preferred form of the invention, polymerizationagent employed in the process comprising a fluorosurfactant tofluoropolyether weight ratio of 5:1 has a surface tension in water at aconcentration of 6000 ppm at 23° C. of at least about 30% less than thesurface tension of the fluorosurfactant alone in water at 23° C. at aconcentration of 6000 ppm.

A preferred class of fluorosurfactants is fluoroether acids or salts,i.e., wherein n is 1 in Formula I above. Preferred fluoroether acids orsalts in the accordance with the invention are fluorosurfactantsaccording to Formula I wherein:

R¹ is a linear or branched partially or fully fluorinated alkyl grouphaving 1 to 3 carbon atoms which may contain ether linkages, and

L is an alkylene group selected from —CX(R²)—, wherein R² is fluorine orperfluoromethyl and X is hydrogen or fluorine, and —CZ¹Z²CZ³Z⁴—, whereinZ¹, Z², Z³, and Z⁴ are independently selected from hydrogen or fluorine.

Fluoroether acids and salts of this type are known. When L is analkylene group selected from —CX(R²)—, wherein R² is fluorine orperfluoromethyl and X is hydrogen or fluorine, the compound can be made,for example, by hydrolysis of perfluoro-2-alkoxypropionyl fluorideintermediates prepared by reacting alkanoic acid fluorides withhexafluoropropylene epoxide as disclosed in U.S. Pat. No. 3,291,843 toFritz and Selman for use in perfluoro(alkyl vinyl ether) manufacture.When L is —CZ₂CZ₂—, wherein Z is independently selected from hydrogen orfluorine, routes to making such compounds are generally described In U.S. Patent 2,713, 593 (Brice et al.) where fluoro(alkoxypropionic) acidsand derivatives are obtained in useful yields from correspondinghydrocarbon alkoxypropionic acids and derivatives by electrochemicalfluorination. Fully fluorinated and partially fluorinated products canbe separated for example by fractional distillation. Useful teachingsfor synthesis can also be found in EP 0 148 482 B1 (Ohsaka et al.) forpartially fluorinated propoxy propionic acid fluoride which may befurther fluorinated or perfluorinated by electrofluorinating the acidfluoride which is then easily converted to an acid or salt.

In accordance with another preferred form of the invention, L in FormulaI is an alkylene group selected from —CF(CF₃)—, —CF₂—, —CF₂CF₂—,—CHFCF₂—, and —CF₂CHF—.

The fluorosurfactant used in accordance with the invention may be adiether if R1 or L contains an ether linkages. Such compounds are made,for example, by the teachings in WO 01/46116 A1 (Hintzer et al.)Preferred fluoroether acids or salts are fluoromonoethers where R¹ and Ldo not contain ether linkages.

In accordance with another preferred form of the invention, R¹ inFormula I is a linear partially or fully fluorinated alkyl group having2 to 3 carbon atoms. Preferably, R¹ is fully fluorinated.

In accordance with another preferred form of the invention, thefluorosurfactant is highly fluorinated. “Highly fluorinated” means thatat least about 50% of the total number of fluorine and hydrogen atoms inthe fluorosurfactant are fluorine atoms. More preferably, at least about75% of the total number of fluorine and hydrogen atoms in thefluorosurfactant are fluorine atoms, most preferably at least about 90%.Perfluorinated surfactants are also preferred for use in accordance withthe invention.

In accordance with one preferred embodiment of the invention, thefluorosurfactant is a compound of the formula:

[CF₃CF₂CF₂OCF(CF₃)COO⁻]Y⁺  (II)

wherein Y⁺ is hydrogen, ammonium, or alkali metal cation. This is acompound is represented by Formula I wherein R¹ is CF₃CF₂CF₂—; L is—CF(CF₃)—; A⁻ is carboxylate; and Y⁺ is hydrogen, ammonium or alkalimetal cation. Preferably, Y⁺ is hydrogen or ammonium. A compound of thisformula can be obtained from the perfluoro-2-propoxypropionyl fluorideintermediate prepared according to U.S. Pat. No. 3,291,843 ordimerization of hexafluoropropylene epoxide and subsequent hydrolysis ofthe resulting acid fluoride to carboxylic acid in the case of the acidand, in the case of the salt, by simultaneous or subsequent reactionwith the appropriate base to the produce the desired salt. A procedurefor dimerization of hexafluoropropylene epoxide is disclosed in G.B.Patent 1 292 268.

In accordance with another preferred embodiment of the invention, thefluorosurfactant is a compound of the formula:

[CF₃CF₂OCF(CF₃)COO⁻]Y⁺  (Ill)

wherein Y⁺ is hydrogen, ammonium, or alkali metal cation. A compound ofthis formula can be obtained from the perfluoro-2-ethoxypropionylfluoride intermediate prepared according to U.S. Pat. No. 3,291,843 andsubsequent hydrolysis of the resulting acid fluoride to carboxylic acidin the case of the acid and, in the case of the salt, by simultaneous orsubsequent reaction with the appropriate base to the produce the desiredsalt.

In accordance with another preferred embodiment of the invention, thefluorosurfactant is a compound of the formula:

[CF₃CF₂CF₂OCF₂CF₂COO⁻]Y⁺  (IV)

wherein Y⁺ is hydrogen, ammonium, or alkali metal cation. A compound ofthis formula can be made, for example, by the teachings of U.S. Pat. No.2,713,593 (Brice et al.).

In accordance with another preferred form of the invention, thefluorosurfactant is a compound of Formula I wherein n is 0 and R¹; Lcollectively comprises a perfluoroalkyl group having 4-6 carbons; and A⁻is sulfonate and sulfonamide anion. In a preferred embodiment of thisform of the invention, A⁻ is sulfonamide anion, the sulfonamide compoundof Formula V below:

[C₄F₉SO₂N⁻CH₂CH₂OH]Y⁺  (V)

wherein Y⁺ is hydrogen, ammonium, or alkali metal cation.

A surfactant of this formula as the ammonium salt is availablecommercially from 3M under the trademark NOVEC™ 4200.

In accordance with another preferred embodiment of the invention, thefluorosurfactant is a compound of the formula:

[CF₃CF₂CF₂CF₂CH₂CH₂SO₂ ⁻]Y⁺

wherein Y⁺ is hydrogen, ammonium, or alkali metal cation.Fluoropolyether Acid or Salt The other component of the polymerizationagent combination used in the practice of the present invention is afluoropolyether acid or salt thereof. Preferably, the fluoropolyether isa perfluoropolyether acid or salt thereof. The acid groups of thefluoropolyether acid or salt thereof preferably are acid groups selectedfrom carboxylic acid, sulfonic acid, sulfonamide, phosphonic acid. Inpreferred embodiments, the acid group of the fluoropolyether acid orsalt is carboxylic acid. Preferably, the fluoropolyether acid isemployed as a salt during polymerization, most preferably, an ammoniumsalt.

Preferred perfluoropolyether (PFPE) acids or salts thereof for use inaccordance with the present invention can have any chain structure inwhich oxygen atoms in the backbone of the molecule are separated bysaturated fluorocarbon groups having 1-3 carbon atoms. More than onetype of fluorocarbon group may be present in the molecule.

Representative structures have the repeat unit represented in thefollowing formulas:

(—CFCF₃—CF₂—O—)_(n)  (VI)

(—CF₂—CF₂—CF₂—O—)_(n)  (VII)

(—CF₂—CF₂—O—)_(n)—(—CF₂—O—)_(m)  (VIII)

(—CF₂—CFCF₃—O—)_(n)—(—CF₂—O—)_(m)  (IX)

These structures are discussed by Kasai in J. Appl. Polymer Sci. 57, 797(1995). As disclosed therein, such PFPE can have a carboxylic acid groupor salt thereof at one end or at both ends. Similarly, such PFPE mayhave a sulfonic acid or phosphonic acid group or salt thereof at one endor both ends. In addition, PFPE with acid functionality at both ends mayhave a different group at each end. For monofunctional PFPE, the otherend of the molecule is usually perfluorinated but may contain a hydrogenor chlorine atom. PFPE having an acid group at one or both ends for usein the present invention has at least 2 ether oxygens, preferably atleast 4 ether oxygens, and even more preferably at least 6 etheroxygens. Preferably, at least one of the fluorocarbon groups separatingether oxygens, and more preferably at least two of such fluorocarbongroups, have 2 or 3 carbon atoms. Even more preferably, at least 50% ofthe fluorocarbon groups separating ether oxygens have 2 or 3 carbonatoms. Also, preferably, the PFPE has a total of at least 15 carbonatoms, e.g., the preferred minimum value of n or n+m in the above repeatunit structures is at least 5. More than one PFPE having an acid groupat one or both ends can be used in a process in accordance with theinvention. Typically, unless extraordinary care is employed tomanufacture a single specific PFPE compound, the PFPE may containmultiple compounds in varying proportions within a molecular weightrange about the average molecular weight.

The fluoropolyether acid or salt thereof has an average molecular weightwhich enables it to function in combination with fluoromonoether acid orsalt as a polymerization agent in a process in accordance with thepresent invention. The number average molecular weight of thefluoropolyether acid or salt employed in accordance with the presentinvention is greater than about 800 g/mol. Fluoropolyether acids orsalts with a number average molecular weight of greater than about 800g/mol are defined in this patent application to be “polymericfluoropolyethers”. The number average molecular weight of thefluoropolyether acids or salts employed is usually less than about 6000g/mol because fluoropolyether acids or salt with very high molecularweights generally are difficult to disperse in the aqueouspolymerization medium. More preferably, the fluoropolyether acid or saltthereof employed in accordance with the invention has a number averagemolecular weight of about 800 to about 3500 g/mol, and most preferably1000 to about 2500 g/mol.

Process

In the practice of a preferred embodiment of the invention, the processis carried out as a batch process in a pressured reactor. Suitablevertical or horizontal reactors for carrying out the process of theinvention are equipped with stirrers for the aqueous medium to providesufficient contact of gas phase monomers such as TFE for desirablereaction rates and uniform incorporation of comonomers if employed. Thereactor preferably includes a cooling jacket surrounding the reactor sothat the reaction temperature may be conveniently controlled bycirculation of a controlled temperature heat exchange medium.

In a typical process, the reactor is first charged with deionized anddeaerated water of the polymerization medium and fluoropolyether acid orsalt and fluorosurfactant are dispersed in the medium. The dispersing ofthe fluoropolyether acid or salt and fluorosurfactant are discussed inmore detail hereinafter. For PTFE homopolymer and modified PTFE,paraffin wax as stabilizer is often added. A suitable procedure for PTFEhomopolymer and modified PTFE includes first pressurizing the reactorwith TFE. If used, the comonomer such as HFP or perfluoro (alkyl vinylether) is then added. A free-radical initiator solution such as ammoniumpersulfate solution is then added. For PTFE homopolymer and modifiedPTFE, a second initiator which is a source of succinic acid such asdisuccinyl peroxide may be present in the initiator solution to reducecoagulum. Alternatively, a redox initiator system such as potassiumpermanganate/oxalic acid is used. The temperature is increased and, oncepolymerization begins, additional TFE is added to maintain the pressure.The beginning of polymerization is referred to as kick-off and isdefined as the point at which gaseous monomer feed pressure is observedto drop substantially, for example, about 10 psi (about 70 kPa).Comonomer and/or chain transfer agent can also be added as thepolymerization proceeds. For some polymerizations, additional monomers,initiator and or polymerization agent may be added during thepolymerization.

Batch dispersion polymerizations can be described as proceeding in twophases. The initial period of the reaction can be said to be anucleation phase during which a given number particles are established.Subsequently, it can be said that a growth phase occurs in which thepredominant action is polymerization of monomer on established particleswith little or no formation of new particles. The transition from thenucleation to the growth phase of polymerization occurs smoothly,typically between about the 4 and about the 10 percent solids in for thepolymerization of TFE.

After batch completion (typically several hours) when the desired amountof polymer or solids content has been achieved, the feeds are stopped,the reactor is vented and purged with nitrogen, and the raw dispersionin the vessel is transferred to a cooling vessel.

The solids content of the dispersion upon completion of polymerizationcan be varied depending upon the intended use for the dispersion. Forexample, the process of the invention can be employed to produce a“seed” dispersion with low solids content, e.g., less than 10%, which isemployed as “seed” for a subsequent polymerization process to a highersolids level. In other processes, the solids content of fluoropolymerdispersion produced by the process of the invention is preferably atleast about 10 wt %. More preferably, the fluoropolymer solids contentis at least about 20 wt %. A preferred range for fluoropolymer solidscontent produced by the process is about 20 wt % to about 65 wt %, evenmore preferably about 20 wt % to about 55 wt %, most preferably, about35 wt % to about 55 wt %.

In a preferred process of the invention, polymerizing produces less thatabout 10 wt %, more preferably less than 3 wt %, even more preferablyless than 1 wt %, most preferably less that about 0.5 wt % undispersedfluoropolymer (coagulum) based on the total weight of fluoropolymerproduced.

The as-polymerized dispersion can be stabilized with anionic, cationic,or nonionic surfactant for certain uses. Typically however, theas-polymerized dispersion is transferred to a dispersion concentrationoperation which produces concentrated dispersions stabilized typicallywith nonionic surfactants by known methods. Solids contents ofconcentrated dispersion is typically about 35 to about 70 wt %. Certaingrades of PTFE dispersion are made for the production of fine powder.For this use, the dispersion is coagulated, the aqueous medium isremoved and the PTFE is dried to produce fine powder.

The dispersion polymerization of melt-processible copolymers is similarexcept that comonomer in significant quantity is added to the batchinitially and/or introduced during polymerization. Chain transfer agentsare typically used in significant amounts to decrease molecular weightto increase melt flow rate. The same dispersion concentration operationcan be used to produce stabilized concentrated dispersions.Alternatively, for melt-processible fluoropolymers uses as moldingresin, the dispersion is coagulated and the aqueous medium is removed.The fluoropolymer is dried then processed into a convenient form such asflake, chip or pellet for use in subsequent melt-processing operations.

The process of the invention may also be carried out as a continuousprocess in a pressurized reactor. A continuous process is especiallyuseful for the manufacture of fluorocarbon elastomers.

Initiators

Polymerization in accordance with the invention employs free radicalinitiators capable of generating radicals under the conditions ofpolymerization. As is well known in the art, initiators for use inaccordance with the invention are selected based on the type offluoropolymer and the desired properties to be obtained, e.g., end grouptype, molecular weight, etc. For some fluoropolymers such asmelt-processible TFE copolymers, water-soluble salts of inorganicperacids are employed which produce anionic end groups in the polymer.Preferred initiators of this type have a relatively long half-life,preferably persulfate salts, e.g., ammonium persulfate or potassiumpersulfate. To shorten the half-life of persulfate initiators, reducingagents such as ammonium bisulfite or sodium metabisulfite, with orwithout metal catalyst salts such as Fe, can be used. Preferredpersulfate initiators are substantially free of metal ions and mostpreferably are ammonium salts.

For the production of PTFE or modified PTFE dispersions for dispersionend uses, small amounts of short chain dicarboxylic acids such assuccinic acid or initiators that produce succinic acid such asdisuccinic acid peroxide (DSP) are preferably also added in addition tothe relatively long half-life initiators such as persulfate salts. Suchshort chain dicarboxylic acids are typically beneficial in reducingundispersed polymer (coagulum). For the production of PTFE dispersionfor the manufacture of fine powder, a redox initiator system such aspotassium permanganate/oxalic acid is often used.

The initiator is added to the aqueous polymerization medium in an amountsufficient to initiate and maintain the polymerization reaction at adesired reaction rate. At least a portion of the initiator is preferablyadded at the beginning of the polymerization. A variety of modes ofaddition may be used including continuously throughout thepolymerization, or in doses or intervals at predetermined times duringthe polymerization. A particularly preferred mode of operation is forinitiator to be precharged to the reactor and additional initiator to becontinuously fed into the reactor as the polymerization proceeds.Preferably, total amounts of ammonium persulfate and/or potassiumpersulfate employed during the course of polymerization are about 25 ppmto about 250 ppm based on the weight of the aqueous medium. Other typesof initiators, for example, potassium permanganate/oxalic acidinitiators, can be employed in amounts and in accordance with proceduresas known in the art.

Chain Transfer Agents

Chain-transfer agents may be used in a process in accordance with theinvention for the polymerization of some types of polymers, e.g., formelt-processible TFE copolymers, to decrease molecular weight for thepurposes of controlling melt viscosity. Chain transfer agents useful forthis purpose are well-known for use in the polymerization of fluorinatedmonomers. Preferred chain transfer agents include hydrogen, aliphatichydrocarbons, halocarbons, hydrohalocarbons or alcohol having 1 to 20carbon atoms, more preferably 1 to 8 carbon atoms. Representativeexamples of such chain transfer agents are alkanes such as ethane,chloroform, 1,4-diiodoperfluorobutane and methanol.

The amount of a chain transfer agent and the mode of addition depend onthe activity of the particular chain transfer agent and on the desiredmolecular weight of the polymer product. A variety of modes of additionmay be used including a single addition before the start ofpolymerization, continuously throughout the polymerization, or in dosesor intervals at predetermined times during the polymerization. Theamount of chain train transfer agent supplied to the polymerizationreactor is preferably about 0.005 to about 5 wt %, more preferably fromabout 0.01 to about 2 wt % based upon the weight of the resultingfluoropolymer.

Polymerization Agent

In accordance with the invention, the fluoropolyether acid or saltthereof and the fluorosurfactant is preferably dispersed adequately inaqueous medium to function effectively as a polymerization agent.“Dispersed” as used in this application refers to either dissolved incases in which the fluoropolyether acid or salt and/or thefluorosurfactant are soluble in the aqueous or dispersing in cases inwhich the fluoropolyether acid or salt and/or the fluorosurfactant arenot fully soluble and are present in very small particles, for exampleabout 1 nm to about 1 μm, in the aqueous medium. Similarly, “dispersing”as used in this application refers to either dissolving or dispersingthe fluoropolyether acid or salt and/or the fluorosurfactant so thatthey are dispersed as defined above. Preferably, the fluoropolyetheracid or salt and fluorosurfactant are dispersed sufficiently so that thepolymerization medium containing fluoropolyether acid or salt andfluorosurfactant appears water clear or nearly water clear. Morepreferably, an aqueous concentrate of the dispersed fluoropolyether acidsalt and fluorosurfactant adjusted to contain 1500 ppm±100 ppm of thefluoropolyether acid or salt has a haze in the test method describedhereinafter of less than about 10% and most preferably less than about7%. A preferred range for the haze of the aqueous concentrate of thedispersed fluoropolyether acid or salt is from about 0 to about 10%.

Low haze values at 1500 ppm±100 ppm for the dispersed fluoropolyetheracid or salt correlate with performance of the polymerization agent inthe aqueous polymerization process, e.g., polymerizations employinglower haze concentrates produce less undispersed polymer (coagulum) thanconcentrates with higher haze values. Typically, the concentration ofthe fluorosurfactant in such concentrates does not affect the haze valueas significantly as the fluoropolyether acid or salt, so concentrationsfor the dispersed fluoropolyether acid or salt of 1500 ppm±100 ppm areused for haze measurements rather than measuring haze at a selectedconcentration of the polymerization agent containing bothfluoropolyether acid or salt thereof and the fluorosurfactant. Hazevalues of the aqueous polymerization medium itself containing thedispersed fluoropolyether salt are less sensitive to the contribution ofhaze by the fluoropolyether salt because of the low fluoropolyether saltcontent and may be affected by other components in the aqueouspolymerization medium.

Dispersing of the fluoropolyether acid or salt thereof and thefluorosurfactant can be carried out in variety of methods. In onesuitable procedure, the polymerization agent can be made directly in theaqueous polymerization medium. In this procedure, the fluoropolyetheracid or salt is supplied in acid form and subsequently converted to saltform. This is accomplished by first adding ammonium hydroxide or alkalimetal hydroxide, preferably ammonium hydroxide, to the aqueouspolymerization medium in a quantity sufficient to substantiallycompletely convert to salt form the subsequently added fluoropolyetheracid. The fluoropolyether acid can then be added to the ammoniumhydroxide or alkali metal hydroxide solution and, if desired, pHmeasurements can be made to determine if insufficient or excess base hasbeen used. In addition, as known in the art, the amount of ammoniumhydroxide or alkali metal hydroxide used, together with other materialsadded to the polymerization medium, should provide a pH in the aqueouspolymerization medium which promotes the desired level of activity forthe particular initiator system used and provides an operable pH rangefor the polymerization agent. The fluorosurfactant can be added to theaqueous polymerization medium prior to, simultaneously with orsubsequently to the addition of the fluoropolyether acid.

In accordance with a preferred form of the process of the invention, theprocedure for making the polymerization agent employs making an aqueousconcentrate of the dispersed fluoropolyether acid or salt andfluorosurfactant which is subsequently added to a larger volume ofaqueous polymerization medium. The concentrate can be made by reactingthe fluoropolyether acid with a small volume of aqueous ammoniumhydroxide or alkali metal hydroxide to produce the concentratecontaining the salt form of the fluoropolyether acid. The surfactant isthen added to the concentrate, preferably in the form of an ammonium oralkali metal salt. Alternatively, the reacting of the fluoropolyetheracid with a small volume of aqueous ammonium hydroxide or alkali metalhydroxide to make the concentrate is done in the presence of thesurfactant.

An appropriate quantity of the concentrate of the dispersedfluoropolyether acid or salt and fluorosurfactant is then mixed into theaqueous polymerization medium to supply the already dispersedfluoropolyether acid or salt and fluorosurfactant in the desiredquantity. Preferably, the amount ammonium hydroxide or alkali metalhydroxide used to make the concentrate should provide a pH in theaqueous polymerization medium which promotes the desired level ofactivity for the particular initiator system used and provides anoperable pH range for the polymerization agent.

In a preferred form of the invention, dispersing aids are used to assistwith dispersing of the fluoropolyether acid or salt by contacting theacid or salt with the dispersing aid. A dispersing aid is especiallyuseful for dispersing higher molecular weight fluoropolyether acid orsalt thereof, e.g., above about 1200 g/mol. Dispersing aids are usefulfor either procedure discussed above for dispersing the fluoropolyetheracid or salt, i.e., for dispersing into the polymerization medium ordispersing into the concentrate. Preferably, fluoropolyether acid orsalt is contacted with the dispersing aid prior to dispersing thefluoropolyether acid or salt in the aqueous medium of the concentrate.Fluorosurfactant can be added after the fluoropolyether acid or salt isdispersed.

Any of a variety of dispersing aids may be used to aid in dispersing thefluoropolyether acid or salt for use in accordance with the presentinvention. A surfactant, preferably the fluorosurfactant to be used inpolymerization, is useful to disperse fluoropolyether acid or salt. Ingeneral, and particularly when polymerizing a high molecular weightfluoropolymer, a low telogenic or non-telogenic dispersing aid ispreferred. With some dispersing aids, it is desirable to mix thedispersing aid with fluoropolyether acid or salt prior to addition tothe aqueous polymerization medium or to the aqueous medium that formsthe concentrate.

One suitable class of dispersing aids includes C3 to C8 alcohols with aparticularly suitable dispersing aid being t-butanol. When thefluoropolyether acid or salt is supplied in acid form and the ammoniumsalt is to be used in the polymerization agent, concentrates can beformed by simultaneously mixing fluoropolyether acid, t-butanol, and anaqueous ammonium hydroxide solution and stirring. Fluoromonoether acidor salt to form the effective polymerization agent combination can beadded subsequently. Preferably, t-butanol is added in an amount of about0.5× to about 3× the weight of the fluoropolyether acid although thelowest amount which is effective is preferably employed to decreasetelogenic effects. C3 to C8 alcohols such as t-butanol would generallynot be used for polymerization of PTFE or modified PTFE because theirtelogenic activity may interfere with achieving the high molecularweight usually desired. In some cases, it is desirable for water to bepresent with the C3 to C8 alcohol, i.e., an alcohol/water mixture isused, to effectively disperse the fluoropolyether acid or salt.

In accordance with another preferred form of the invention, thefluoropolyether acid or salt is supplied in acid form and also thesurfactant to be used is also be supplied in acid form (Y⁺ in Formula Iis H). It has been discovered that the fluoropolyether acid andfluorosurfactant acid form will form a mixture which can readily bedispersed into an aqueous medium, i.e., the aqueous polymerizationmedium or aqueous solution to make the concentrate. In addition, theacid mixture is readily dispersed even when the fluoropolyethercarboxylic acid has a high molecular weight and may otherwise require adispersing aid. This preferred procedure is especially useful in theproduction of PTFE or modified PTFE where the telogenic effects ofdispersing aids such as t-butanol may make it difficult to achieve adesired high molecular weight and would adversely affect the use of wax.In this preferred form of the invention, the fluoropolyether acid andfluorosurfactant in acid form are mixed together to form an acid mixtureprior to addition to the aqueous polymerization medium or a concentrate.Preferably, the mixture of the fluoropolyether acid and fluorosurfactantcomprises less than about 50 wt % water. In a preferred form of theinvention which employs the polymerization agent in salt form, this acidmixture is contacted with ammonium hydroxide or alkali metal hydroxide.More preferably, this mixture is contacted with an aqueous ammoniumhydroxide solution to form dispersed fluoropolyether salt andfluorosurfactant in ammonium salt form. In one preferred form of theinvention, the contacting of the acid mixture with ammonium hydroxide oralkali metal hydroxide is performed by providing the ammonium hydroxideor alkali metal hydroxide in the polymerization medium and mixing theacid mixture into the polymerization medium to disperse the salts of thefluoropolyether acid and fluorosurfactant into the polymerizationmedium. In another preferred form of the process, the contacting of theacid mixture with ammonium hydroxide or alkali metal hydroxide isperformed using aqueous ammonium hydroxide or aqueous alkali metalhydroxide to form an aqueous concentrate comprising dispersed salts ofthe fluoropolyether acid and fluorosurfactant. The aqueous concentrateis added to the aqueous polymerization medium to disperse the salts ofthe fluoropolyether acid and fluorosurfactant in the polymerizationmedium.

For the manufacture of concentrates containing dispersed fluoropolyetheracid or salt and fluorosurfactant, aqueous ammonium hydroxide or aqueousalkali metal hydroxide solutions are preferably employed. Ammonium saltsare preferred for the practice of this invention and thus aqueousammonium hydroxide is preferably used. The aqueous ammonium hydroxideemployed preferably has an ammonia content of about 15 to about 40 wt %.

To facilitate manufacture of the polymerization agent concentrates, isit sometimes desirable to first make a concentrate with a very highamount of the polymerization agent, e.g., 5000 to 500,000 ppm dispersedfluoropolyether acid or salt. Although this “superconcentrate” can beintroduced directly into the polymerization medium, it is preferablydiluted with a modest amount (10 to 100 volumes) of water first to makea concentrate which provides the dispersed polymerization agent to theaqueous polymerization medium. In adding the superconcentrate to thedilution water, it is preferred that the water be vigorously stirred andthe concentrate be added slowly, dropwise or through a capillary orsmall diameter tube immersed in the stirred water at a point of highshear. If the superconcentrate is introduced into the polymerizationvessel undiluted, the addition to water in the polymerization vesselpreferably should similarly be done gradually with vigorous mixing.Vigorous mixing is generally desirable at all stages of manufacture ofpolymerization agent concentrates and introduction into the aqueouspolymerization medium.

Preferred aqueous concentrates employed in accordance with the inventioncomprise about 1 to about 95 wt % water, most preferably about 50 toabout 95 wt % water. Preferably, the aqueous concentrate comprises about0.5 to about 50 wt % dispersed fluoropolyether acid or salt. Preferredconcentrates are stable at room temperature, i.e., such concentrates canstand for at least one week at room temperature without any substantialquantity of the fluoropolyether acid or salt and fluorosurfactantseparating from the concentrate.

In a preferred form of the invention, the polymerization agent employedin accordance with the present invention comprises a major amount byweight of fluorosurfactant and a minor amount by weight offluoropolyether acid or salt thereof. More preferably, thefluorosurfactant comprises at least about 55 wt % of the polymerizationagent, most preferably, at least about 65 wt % of the polymerizationagent. This form of the invention can be used with variousfluoropolymers but is particularly useful for the polymerization of PTFEor modified PTFE having a comonomer content of no greater than about 1wt %. In another preferred form of the invention, the polymerizationagent comprises a major amount of said fluoropolyether acid or saltthereof and a minor amount of fluorosurfactant. This form of theinvention is useful for melt-processible copolymers such as copolymerscomprising at least about 60-98 wt % tetrafluoroethylene units and about2-40 wt % of at least one other monomer. It is generally desirable forthe exact proportion of fluorosurfactant to the fluoropolyether acid orsalt in the polymerization agent to be adjusted depending upon thefluorosurfactant used, the molecular weight of the fluoropolyether acidor salt used, the intended properties for the fluoropolymer dispersion,etc.

Preferably, the amount of fluoropolyether acid or salt used in theaqueous polymerization medium is about 5 to about 3,000 ppm based on theweight of water in the aqueous polymerization medium. More preferably,the amount of fluoropolyether acid or salt used in the aqueouspolymerization medium is about 5 to about 2000 ppm based on the weightof water in the aqueous polymerization medium, even more preferablyabout 50 to about 1000 ppm, and most preferably about 100 to about 350ppm. The total amount of polymerization agent combination used in apreferred process in accordance with the invention is about 5 to about10,000 ppm based on the weight of water in the aqueous medium, morepreferably about 5 to about 3000 ppm based on the weight of water in theaqueous medium. Even more preferably, the total amount of polymerizationagent combination used is about 50 to about 3000 ppm based on the weightof water in the aqueous medium, still more preferably about 50 ppm toabout 2000 ppm, most preferably, about 150 ppm to about 500 ppm.

At least a portion of the polymerization agent is preferably added tothe polymerization prior to the beginning of the polymerization. Ifadded subsequently, a variety of modes of addition for thepolymerization agent may be used including continuously throughout thepolymerization, or in doses or intervals at predetermined times duringthe polymerization. In accordance with one embodiment of the invention,substantially all of the polymerization agent is added to the aqueousmedium prior to the start of polymerization, preferably prior toinitiator addition.

In accordance with the invention, the aqueous medium comprises less thanabout 300 ppm of perfluoroalkane carboxylic acid or saltfluorosurfactants having 8 or more carbon atoms, based on the weight ofwater in the aqueous medium. Perfluoroalkane carboxylic acid or saltfluorosurfactants having 8 or more carbon atoms include such surfactantshaving for example 8-14 carbon atoms, e.g., perfluorooctanoic acid andsalts and perfluorononanoic acid and salts. Preferably, the aqueousmedium comprises less than about 100 ppm of perfluoroalkane carboxylicacid or salt fluorosurfactants having 8 or more carbon atoms, morepreferably less than 50 ppm. In a preferred embodiment of the invention,the aqueous medium is substantially free of perfluoroalkane carboxylicacid or salt fluorosurfactants having 8 or more carbon atoms.Substantially free of perfluoroalkane carboxylic acid or saltfluorosurfactants having 8 or more carbon atoms means that aqueousmedium contains no more than about 10 ppm of such fluorosurfactants.

In accordance with a preferred form of the invention the polymerizationagent combination used in the practice of this invention is preferablysubstantially free of perfluoropolyether oil (i.e., perfluoropolyethershaving neutral, nonionic, preferably fluorine or hydrogen, end groups).Substantially free of perfluoropolyether oils means that aqueouspolymerization medium contains no more than about 10 ppm of such oilsbased on water. This form of the invention is unlike the aqueousmicroemulsion system as disclosed in U.S. Pat. No. 4,864,006 to Gianettiet al., which employs such perfluoropolyether oils. Thus, thefluoropolymer dispersion preferably produced has high purity andcontains low residual surfactant and preferably is substantially free ofperfluoropolyether oils. Moreover, in a preferred process, thepolymerization medium is substantially free of fluoropolymer seed atpolymerization kick-off. In this preferred form of the invention,fluoropolymer seed, i.e., separately polymerized small fluoropolymerparticles in dispersion form, is not added prior to the start ofpolymerization.

Unexpectedly, it has been found that a fluoropolyether acid or salthaving a number average molecular weight of at least about 800 g/mol incombination with a fluorosurfactant having a chain length of no greaterthan 6 provides an effective polymerization agent for use in themanufacture of fluoropolymers; whereas, fluorosurfactants having a chainlength of no greater than 6 when used alone do not produce the desiredlow coagulum levels at fluoropolymer solids concentrations forcommercial fluoropolymer manufacture. The polymerization agentcombination in accordance with the invention can produce fluoropolymersequivalent to those made using the typical perfluoroalkane carboxylicacid surfactants disclosed in U.S. Pat. No. 2,559,752 to Berry and athigh dispersion solids concentrations.

TEST METHODS

The melting point (Tm) and glass transition temperature (Tg) ofcopolymers is measured by Differential Scanning Calorimeter according tothe procedure of ASTM D 4591. PTFE homopolymer melting point, themelting point the first time the polymer is heated, also referred to asthe first heat, is determined by differential scanning calorimetry (DSC)by the method of ASTM D-4591-87. The melting temperature reported is thepeak temperature of the endotherm on first melting.

Standard specific gravity (SSG) is measured by the method of ASTMD-4895.

Comonomer content (PPVE or HFP) is measured by FTIR according to themethod disclosed in U.S. Pat. No. 4,743,658, col. 5, lines 9-23.

Comonomer content (PDD) is measured by IR by comparing the absorbanceratio at 2404 cm⁻¹ to 1550 cm⁻¹ to a calibration curve

Melt flow rate (MFR) is measured according to ASTM D-1238 at thetemperature which is standard for the specific copolymer.

Haze is measured on an aqueous concentrate of the dispersedfluoropolyether acid or salt and fluorosurfactant (and dispersing aid,if used) which is adjusted to contain 1500 ppm±100 ppm of thefluoropolyether acid or salt. The haze is measured in transmission modeon a Hunter® ColorQuest XE spectrophotometer with sphere geometry usingHunterLab Universal Software v 4.0. The sample cell is a 50 mmtransmission cell. The transmission haze measurement is the ratio ofdiffused light to the total light transmitted by a specimen multipliedby 100 to express a percentage of transmission.

Surface Tension is determined using the Wilhelmy Plate Method on aKruess Tensiometer, K11-MK2.

SURFACE TENSION EXAMPLES

Surface tension of selected short chain and long chain surfactants aremeasured (1) as ammonium salts in water and (2) by combining thesurfactants in water with a perfluoropolyether carboxylic acid having anumber average molecular weight of about 2100 (n=about 12 in FormulaVI). The perfluoropolyether carboxylic acid is available commerciallyavailable as Krytox® 157 FSL from DuPont. This perfluoropolyethercarboxylic acid (referred to as PFPEA 2) is also employed in thePolymerization Examples illustrating polymerization in accordance withthe invention.

In general the surfactant, if in acid form, is added to water withsubsequent addition of aqueous ammonium hydroxide (30 wt % aqueoussolution, wt % calculated as NH₃) and stirred either manually or with amagnetic stirrer. Additional water is added so that the surfactantconcentration is 6000 ppm based on water with additional stirring asindicated. If the surfactant is in the form of an ammonium salt inwater, the surfactant salt is diluted and stirred without adding moreammonium hydroxide. Sufficient water is added so that the surfactantconcentration is 6000 ppm based on water.

Preparation of samples of the surfactant combined withperfluoropolyether carboxylic acid in a 5:1 weight ratio by thesequential addition of the PFPEA 2, water, aqueous ammonium hydroxide,surfactant, additional water as needed and stirred either manually orwith a magnetic stirrer. The visual appearance of the samples isgenerally clear with haze less than 5%. Sufficient water is added sothat the surfactant concentration is 6000 ppm based on water.

Surface Tension measurements are made using the Kruess Tensiometer,K11-MK2. Surface tension data is the average of 10 data points taken bythe instrument. Measurements are done at ambient temperature (23° C.).Surface tensions of the surfactant alone and of the surfactant combinedwith PFPEA 2 are reported in Tables A and B. The surfactants with shortchain lengths have surface tension values significantly higher than thesurface tension values of perfluorooctanoic acid and salts and many ofthe other long chain length surfactants under the same conditions. Ashort chain surfactant in combination with the PFPEA 2 has a surfacetension significantly lower than the surfactant used alone, i.e., asurface tension in water at a concentration of 6000 ppm at 23° C. of atleast about 30% less than the surface tension of the fluorosurfactantalone in water at 23° C. at a concentration of 6000 ppm.

TABLE A Short Chain Polymerization Surfactants SURFACE TENSION @ 6000ppm dynes/cm Surf/ CHAIN FSL Example # STRUCTURE LENGTH Surf 5:1 ExampleI

4 53.8 26.3 Example II [C₄F₉—SO₂—N⁻CH₂CH₂OH]NH₄ ⁺ 4 43.7 19.0 ExampleIII —

5 44.5 24.5 Example IV

6 45.3 22.6 Example V C₄F₉CH₂CH₂SO₃ Na 6 42.5 26.9 The surface tensionof deionized water is recorded as 67.9 dynes/cm when surface tensionmeasurements of Examples I-IV are made.

TABLE B Long Chain Polymerization Surfactants SURFACE TENSION @ 6000 ppmDynes/cm Surf/ CHAIN FSL Example # STRUCTURE LENGTH Surf 5:1 Example VI(Comp.)

7 26.8 33.0 Example VII (Comp.)

7 42.4 43.5 Example VIII (Comp.)

7 22.6 36.2 Sample IX C₆F₁₃—CH₂CH₂—SO₃ NH₄ 8 21.2 29.2 (Comp.) Sample X(Comp.)

8 15.4 16.7′ The surface tension of deionized water is recorded as 67.9dynes/cm when surface tension measurements of Comp. Examples VI and VIIare made. The surface tension of deionized water is recorded as 62.5dynes/cm when surface tension measurements of Example VII, IX and X aremade.

POLYMERIZATION EXAMPLES Polymerization Agent Components

In Examples 1-9 and Comparative Examples 5-7, fluoromonoether acid isemployed having the formula CF₃CF₂CF₂OCF(CF3)COOH (referred to as dimeracid or DA) which is converted to the ammonium salt in the exampleswhich follow (referred to as dimer acid salt or DAS).

In Examples 10-14 and Comparative Examples 8-10, afluoroalkylsulfonamide fluorosurfactant is employed having a formulaC₄F₉SO₂NHCH₂CH₂OH. The surfactant is available from 3M SpecialtyMaterials, St Paul, Minn., under the trademark NOVEC™ 4200 in the formof an aqueous ammonium salt, 25 wt %.

Two fluoropolyether acids are employed which are perfluoropolyetheracids having carboxylic acid group (PFPEA) each having the repeat unitof Formula VI above and are converted to ammonium salts in the exampleswhich follow. PFPEA 1 has a number average molecular weight of about1165 (n=about 6 in Formula VI). PFPEA 2 has a number average molecularweight of about 2100 (n=about 12 in Formula VI). PFPEA 2 is availablecommercially available as Krytox® 157 FSL from DuPont.

The ammonium hydroxide in Examples 1-9 and Comparative Examples 5-7 is a30 wt % aqueous solution (wt % calculated as NH₃).

For the Examples 1, 2 and 4 containing PFPEA 1, polymerization agentconcentrates containing PFPEA 1 are made by first adding 510 g ofdeionized water to a 1 liter glass container. The amount of 30 wt %ammonium hydroxide indicated in Table 1 is added to the 510 g ofdeionized water. Then, the amount of PFPEA 1 indicated in Table 1 isadded. The contents of the container are mixed either mechanically orwith ultrasound to produce a slightly cloudy mixture (haze less thanabout 7%). The amount of dimer acid salt (DAS) as indicated in the Table1 below is added. Upon additional mixing, the mixture becomes waterclear.

TABLE 1 PFPEA1 DAS NH₄OH Example (g) (g) (g) Ex 1 1.06 18.9 0.9 Ex 2 1.30.317 1.11 Ex 4 1.3 0.317 1.11

For Example 3, a polymerization agent concentrate containing PFPEA 2 ismade by adding 4.27 g PFPEA 2, 8.54 g t-butanol (dispersing aid), 14.7 gdeionized water, and 0.96 g 30 wt % ammonium hydroxide to a vial whichis sealed and shaken under cold running water to remove the heat ofreaction. A colorless, single phase liquid results. This liquid is addeddropwise with agitation to 878.7 g of deionized water and a clearmixture results (Haze less than about 3%). 0.56 g dimer acid salt (DAS)is added with stirring. The final mixture is water clear (haze less thanabout 3%).

For Examples 5-8, polymerization agent concentrates are prepared bymixing 5 g dimer acid (DA) and 1.3 g PFPEA 2 in a vial. To this mixtureis added dropwise with mild agitation, 1.86 g of aqueous ammoniumhydroxide (30 wt %). This mixture is added dropwise to 900 g deionizedwater with vigorous stirring. The final mixture is water-clear (hazeless than about 3%) and has a pH of 8.5. The dimer acid in theseexamples is functioning as dispersing aid for the PFPEA 2 as well as acomponent of the polymerization agent.

For Example 9, a polymerization agent concentrate is made by mixing14.25 g of dimer acid and 3.71 g of PFPEA 2 in a vial. While cooling thevial in ice, 5.3 g of aqueous ammonium hydroxide (30 wt %) is addeddropwise with mixing. The resulting mixture is water clear, and is addeddropwise to 900 g of deaerated water with stirring to produce awater-clear (haze less than about 3%) solution of pH 9. A second portionof surfactant solution is made the same way. The dimer acid in thisexample is functioning as dispersing aid for the PFPEA 2 as well as acomponent of the polymerization agent.

Examples 1-3

The process of the invention is illustrated in the polymerization ofmelt-processible copolymers of tetrafluoroethylene (TFE) withperfluoro(alkyl vinyl ether), i.e., perfluoro(propyl vinyl ether)(PPVE).

Deaerated water is used in the polymerizations. It is prepared bypumping deionized water into a large stainless steel vessel andvigorously bubbling nitrogen gas for approximately 30 minutes throughthe water to remove all oxygen.

In a 12 liter, horizontal autoclave, equipped with a paddle agitator,7.57 kg of deaerated water is added. 510 ml of the PFPEA 1 or PFPEA 2concentrates described above are charged to the autoclave to provide thepolymerization medium for the Examples. Based on the amounts employed tomake the concentrates, Table 2A shows the amounts of polymerizationagent components in the polymerization medium (ppm based on weight ofwater in the aqueous medium) with initiator, chain transfer agent, andmonomers.

A vacuum of approximately 28 inches of water column (7 kPa) is appliedto the reactor. The reactor is then raised to 30 psig (310 kPa) withgaseous tetrafluoroethylene (TFE) while agitating at 70 rpm. Theagitator is stopped and the TFE pressure reduced to approximately 10psig (100 kPa) by venting. This pressure/vent cycle is conducted twomore times, further insuring that the contents of the autoclave are freeof oxygen. Ethane (0.3-0.5 g) and perfluoro(propyl vinyl ether) (PPVE)(100 g) is then added to the reactor.

The reactor is then heated to 75° C. with agitation at 100 rpm. When attemperature, the reactor pressure is raised to a nominal 300 psig (2.17MPa) by adding TFE (270-330 g). Initiator solution, containing 6.2 gramsof ammonium persulfate in 1 liter of deionized water, is charged to theautoclave at a rate of 100 ml/min to provide a precharge of 0.45-0.74 gammonium persulfate as indicated in Table 2A. The same initiatorsolution is pumped continuously to the autoclave during polymerizationat a rate of 0.54 ml/min. At kickoff (defined as the point at which a 10psig (70 kPa) pressure drop is observed) the polymerization is deemed tohave been started. Reactor pressure is allowed to cycle between 285 psig(2.1 MPa) and 315 psig (2.28 MPa) by intermittently making up monomerscomposed of 96 wt % TFE and 4 wt % PPVE. After the total monomers(including precharged PPVE and TFE) stated in Table 2A is reached, theagitator is stopped and the reactor vented to atmospheric pressure. Thefluoropolymer dispersion thus produced has a solids content of greaterthan 10%. Polymer is isolated from the dispersion by freezing, thawingand filtration. Using a high speed agitator, the polymer is washed indemineralized water and filtered several times before being driedovernight in a vacuum oven at 100 to 110° C. and a vacuum of 6 to 10 mmHg (0.8-1.3 kPa). Results are reported in Table 2B.

Comparative Examples 1-7

Following the general procedure of Examples 1-3, Comparative Examplesare made in which one or the other of PFPEA and DAS is used, but notboth. In Comparative Examples 1-4, only PFPEA is used (PFPEA2). InComparative Examples 5-7, only DAS is used. Table 2A shows the amountsof polymerization agent components in the polymerization medium (ppmbased on weight of water in the aqueous medium) with initiator, chaintransfer agent, and monomers. Polymerization results are summarized inTable 2B. In contrast to Examples 1-3, the Comparative Examples giveproduct that is gelled, or settled, both conditions indicative ofunstable dispersions, or with much undispersed polymer in addition todispersion, also an indication of insufficiently stabilized dispersion.Neither PFPEA nor DAS alone are sufficient to permit aqueous dispersionpolymerization making polymer dispersion to solids greater than about 10wt %.

TABLE 2A TFE/PPVE Polymerization Total PFPEA{circumflex over ( )}PFPEAA{circumflex over ( )} DAS DAS APS Ethane Monomers Example (g)(PPm) (g) (PPm) (g) (g) (g) Ex 1 1.06 130 18.9 2310 0.764 0.5 3096 Ex 21.3 159 0.317 39 0.618 0.5 2024 Ex 3 1.3 159 0.317 39 0.747 0.5 2036Comp. Ex. 1* 1.3 159 0 — 0.609 0 2085 Comp. Ex. 2* 1.3 159 0 — 0.609 02060 Comp. Ex. 3* 6.3 768 0 — 0.620 0 2070 Comp. Ex. 4* 1.615 197 0 —0.499 0.5 1968 Comp. Ex. 5 0 — 1.62 198 0.643 0.5 2065 Comp. Ex. 6 0 —1.617 197 0.499 0.5 1995 Comp. Ex. 7 0 — 1.617 197 0.499 0.5 2031{circumflex over ( )}Examples 1-2 use PFPEA 1. Example 3 and ComparativeExamples 1-4 use PFPEA2. *t-butanol used as dispersing aid in preparingsurfactant mixture.

TABLE 2B TFE/PPVE Polymerization Undispersed Kick-off Completion SolidsM. Pt. PPVE Polymer Example (min) Time (min) wt % (° C.) wt % MFR (g) wt%** Ex 1 4 125 23.7 311 3.88 1.3 11.9 0.5% Ex 2 3 92 15.9 309 4.69 210.58 0.0% Ex 3 1 133 17.0 311 4.46 >50* 7.47 0.4% Comp. Ex 1* 3 98 Gel295, 322 4.10 No flow — — Comp. Ex 2* 2 102 Gel 296, 321 4.76 No flow —— Comp. Ex 3* 2 125 Gel 296, 319 5.59 No flow — — Comp. Ex 4* 1 144 Gel311 4.45 >50 — — Comp. Ex 5 15 88 16.9 310 3.87 9.5 230 12 Comp. Ex 6 1120 Settled 314 3.34 25 1425 g >75 Comp. Ex 7 1 101 settled 313 3.95 14— *t-butanol used as dispersing aid in preparing surfactant mixture.Chain transfer activity appears to have affected melt flow rate. **Basedon polymer.

Example 4

The process of the invention is illustrated in the polymerization of amelt-processible copolymer of tetrafluoroethylene (TFE) withhexafluoropropylene (HFP).

Tetrafluoroethylene-hexafluoropropylene copolymerization is run likethat of tetrafluoroethylene-perfluoro(propyl vinyl ether) Examples 1-2(PFPEA1) with these differences:

Total water (initial charge plus water added with initiator andpolymerization agent) 6540 g. Initiator precharge is 0.66 g of 6.2g/liter water solution. Initial HFP charge is 568 g. Initial TFE chargeis 379 g, as a 60:40 blend of the gases. HFP:TFE feed duringpolymerization is 1200 g/hr of a 12:88 mixture.

Table 3A shows the amounts of polymerization agent components in thepolymerization medium (ppm based on weight of water in the aqueousmedium) with initiator, chain transfer agent, and monomers.Polymerization results are summarized in Table 3B.

TABLE 3A TFE/HFP Polymerization Total PFPEA 1 PFPEA 1 DAS DAS APS EthaneMonomers Example (g) (ppm) (g) (ppm) (g) (g) (g) Ex 4 1.3 199 0.317 483.630 0 2884

TABLE 3B TFE/HFP Polymerization Undispersed Kick-off Completion SolidsMPt. HFP Polymer Example (min) Time (min) wt% (° C.) wt % MFR (g)wt % Ex4 6 89 15.2 264 10.3 55 11.60.9%

Examples 5-7

The process of the invention is illustrated in the polymerization ofpolytetrafluoroethylene (PTFE) homopolymer.

A 12 liter, horizontal autoclave, equipped with a paddle agitator, ispurged with nitrogen, and then a vacuum of about 20 inches Hg (67 kPa)is applied to facilitate the addition of ingredients. After addition ofingredients, the vacuum is applied again. The ingredients are as followsin the order of addition:

-   -   1. Wax (260 g) is melted under nitrogen and is added to the        reactor through a heated addition funnel.    -   2. Deaerated Water, 4375 g.    -   3. Polymerization Agent Concentrate (PFPEA 2/DAS mixture        described above), 900 ml    -   4. Succinic acid, 3 g in 500 ml solution.    -   5. Oxalic acid 15 ml of a 100 ml aqueous solution containing 2 g        oxalic acid.

The concentration of PFPEA 2 is 224 ppm, and that of DAS is 863 ppm,based on the polymerization medium (water) before initiation.

Tetrafluoroethylene (TFE) is added to bring the reactor pressure toabout 30 psig (310 kPa). The reactor is then vented to just aboveatmospheric pressure. This pressure/vent cycle is repeated twice.

The reactor is heated to 80° C., the agitator being started at 65 rpmwhen the temperature reaches 60° C. TFE is added to bring the reactorpressure to about 400 psig (2.9 MPa). This is about 685 g of TFE.

Initiator solution (the “precharge”) is added, 50-80 ml at 100 ml/min,followed by a continuous addition at 1.5 ml/min. The initiator solutionis 0.15 g potassium permanganate (KMnO₄) in 1000 g deaerated watercontaining 5 ml of diammonium phosphate (NH₄)₂HPO₄ solution, which is2.8 g (NH₄)₂HPO₄ in 200 ml water.

About 1 minute after initiator is added, kickoff occurs (defined as thepoint at which a 10 psig (70 kPa) pressure drop is observed) andpolymerization begins. During polymerization TFE is added periodicallyto maintain reactor pressure between 395 and 405 psig (2.7 and 2.8 MPa).After 1100 g of TFE have been consumed, initiator feed is stopped. After2500 g of TFE have been consumed, the agitator is stopped and thereactor is vented. The polymer dispersion is drained from the reactorwhile still hot, and allowed to cool. After cooling the solidified waxis separated and weighed. The reactor is opened and cleaned, thematerial recovered on cleaning being designated undispersed polymer,which is weighed.

The dispersion is weighed and the percent solids is determined. Polymeris recovered by freezing and thawing a portion of the dispersion tocoagulate polymer, and washing and drying the coagulate.

The results are summarized in Table 4. All show polymer at about 30 wt %solids, with melting points characteristic of polytetrafluoroethylene.

TABLE 4 TFE Homopolymerization Total Run- Wax Undispersed Melting TFEtime Dispersion wt. Solids Recovered Polymer Point Ex. (g) (min) (g) (wt%) (g) (g) % Dispersion pH SSG (° C.) 5 2500 215 8202 29.9 119 254 9.4 4345.6 6 2500 286 8248 29.3 153 182 7.0 4 2.177 345.8 7 2500 159 817028.0 162 148 6.1 4 345.4

Example 8

Polymerization conditions are similar to those of Examples 5-7, exceptthat the initiator is changed to disuccinic acid peroxide (DSP) with asmall amount of ammonium persulfate (APS). DSP, 1.5 g, is added in 100ml aqueous solution and 1 ml of a 1 wt % aqueous solution of APS. TFEconsumption is 2000 g. Total run time is 227 min. The polymer dispersionweighs 7906 g and is 26.7% solids. Polymer melting point is 344.7° C.370 g of wax is recovered (this exceeds the wax charged, indicating thatsome coagulum is mixed with the recovered wax). The reactor is notopened so coagulum is not determined.

Example 9

DSP with APS initiator is used as in Example 8, but the horizontalautoclave equipped with a paddle agitator has a capacity of 34.4 liter(approximately 9 gallon). The water charged is 14,700 g, and wax 850 g.1800 ml of the PFPEA 2/DAS polymerization agent concentrate made asdescribed above then charged to the autoclave. The reactor is chargedwith 1716 g of TFE to 375 psig (2.7 MPa). The initiator charge is 340 gof an aqueous solution containing 11.9 g of disuccinic acid peroxide and3.4 ml of a 1 wt % aqueous ammonium persulfate solution. TFE consumptionis 9000 g. Total run time is 223 min. 25,637 g of dispersion isrecovered. Solids is 35.1%. SSG is 2.203. Recovered wax is 578 g.Coagulum is 622 g, 6.5 wt %.

Examples 10-14, Comparative Examples 8-10

The process of the invention is illustrated in the polymerization oftetrafluoroethylene (TFE) and perfluoro(2,2-dimethyl-1,3-dioxole) (PDD)copolymers.

TFE and perfluoro(2,2-dimethyl-1,3-dioxole) (PDD) copolymers areprepared in a 1 L vertical stainless steel jacketed autoclave equippedwith a ribbon agitator with 3 blades about 0.61 cm (¼″) wide and 12.7 cm(5″) long, and 2 vertical baffles 0.61 (¼″) wide that spanned the heightof the reactor. The reactor is conditioned by charging 600 mL deionized(DI) water containing 2.5 g ammonium persulfate (APS) and heating thismixture with vigorous agitation to 90° C. for 3 hrs. The reactor isdrained, flushed with DI water and refilled with 600 mL of fresh DIwater. The reactor and its contents are purged and evacuated withnitrogen three times and 25 mL of polymerization agent, as noted in theTable 5A below, is added. The reactor contents are then warmed to 60° C.and the nitrogen atmosphere in the reactor is evacuated and replacedwith TFE. When thermal equilibrium is established, TFE and PDD flows arefed to the reactor at the precharge ratio specified in the tables.Gaseous TFE feed is monitored and controlled by a mass flow meter andliquid PDD flows are controlled by a Gilson metering pump. Monomer feedsare continued until the desired reactor pressure is attained. Flows arestopped and this completes the monomer precharge. Polymerization isinitiated by injecting 10 mL of DI water containing 1.0 g APS andobserved by a decrease in reactor pressure. At that time monomer flowsat the specified ratio are restarted and at a rate sufficient tomaintain reactor pressure. In examples 12-15 and 17, the reactorpressure is 48 psig. In example 16, the reactor pressure is 30. Flowsare continued until predetermined amounts of monomers are fed to thereactor. At that point flows are stopped and after a brief holdingperiod the reactor is cooled, vented to atmospheric pressure and thepolymer dispersion is discharged. The collected dispersion is weighedand compared to the total weight of all materials charged to thereactor. This mass balance check is useful to determine if coagulationof the polymer has occurred in the reactor. Mass balances less than 90%indicate significant coagulation has occurred.

The above procedure is used to evaluate the polymerization agentcomprising sulfonamide Novec™ 4200 surfactant and the sulfonamide incombination with PFPEA 2 in the emulsion polymerization of TFE and PDD.

Novec™ 4200 surfactant as supplied (25 wt % aqueous solution of theammonium salt) is combined with PFPEA 2. No additional ammoniumhydroxide is added. This mixture is shaken and rolled for several hoursto make a concentrate.

The results are summarized in Tables 5 A and 5B below. The ComparativeExamples are characterized by low solids in the product dispersion, lowpolymer yields and early termination of polymerization reflected by theshort feed times for monomer. The glass transition temperature (Tg) andPDD content of the polymer made in the Comparative Examples are lowerthan runs made under similar conditions in the Examples. The MFR forpolymer made in the Comparative Examples also tends to be so high thatthe polymer runs rapidly out of the melt indexer and precludesmeasurement. These results all indicate that Novec™ 4200 by itself orwith a minimum amount of PFPEA 2 (as shown in Comparative Example 9) isineffective as a polymerization surfactant. Results for runs employingNovec™ 4200 and at least 200 mg PFPEA 2 reliably yield consistentpolymerization results indicating that under these conditions thesemixtures are suitable surfactants for polymerization.

TABLE 5A TFE/PDD Polymerization Novec ™ Novec ™ Precharge Make-up PFPEA2PFPEA2 4200 4200 PDD TFE PDD TFE Ex. (g) (ppm) (g) (PPm) G G Ratio g GRatio 10 300 mg 480 ppm 3.75 g 6000 23.65 4.01 5.90 132.3 22.43 5.90 11300 mg 485 ppm 2.00 g 3200 27.01 4.58 5.90 131.4 22.28 5.90 12 200 mg322 ppm 3.00 g 4800 29.22 4.95 5.90 132.6 22.48 5.90 13 200 mg 316 ppm5.75 g 9100 28.48 4.88 5.84 135.0 23.18 5.82 14 200 mg 316 ppm 5.75 g9100 25.53 4.42 5.78 133.2 23.07 5.77 C- 0 0 ppm  4.0 g 6400 26.15 4.495.82 133.2 22.88 5.82 C- 100 mg 160 ppm 3.75 g 6000 26.89 4.60 5.85132.9 22.54 5.90 C- 0 0 ppm 6.00 g 9500 24.65 4.25 5.80 61.08 10.55 5.79

TABLE 5B TFE/PDD Polymerization Feed Mass Undispersed** Time Tg PDD MFRSolids Polymer Balance Polymer Example (hr:min) (° C.) wt % (g/10 min) %(g) % (g) wt % 10 4:22 154 65.6 0.26 18.1 142.0 96.5 3.1 2.1 11 4:49 16060.5 0.52 18.6 141.2 94.6 6.2 4.2 12 5:10 162 66.9 0.88 18.8 145.3 95.15.3 3.5 13 3:55 153 63.0 0.47 18.7 146.8 94.9 5.6 3.6 14 3:55 158 64.318.4 145.4 96.2 2.8 1.8 C-8 3:51 124 54.8 0.50 11.8 82.9 86.1 65.8 44.3C-9 3:57 139 54.5 v high 8.3 67.2 76.1 81.5 54.8 C-10 2:11 114 50.0 vhigh 6.0 41.0 93.5 39.0 48.8 **Based on polymer and on expected amountof polymer produced from 80% monomer conversion.

Example 15

The process of the invention is illustrated in the polymerization of afluorocarbon elastomer copolymer of tetrafluoroethylene (TFE),perfluoro(methyl vinyl) ether (PMVE), andperfluoro-8(cyano-5-methyl-3,6-dioxa-1-octene) (8CNVE).

Three aqueous streams are each fed continuously to a 1 litermechanically stirred, water jacketed, stainless steel autoclave at arate of 81 cubic centimeters/hour. The first stream consists of 3.4 gammonium persulfate and 89.7 g of disodium hydrogen phosphate in 3liters of deionized water. The second consists of a polymerizationconcentrate of 180 g of DA, 36 g of PFPEA 2, and 35 g 30 wt % ammoniumhydroxide in 4 liter of deionized water. The third stream consists of3.4 g ammonium persulfate in 3 liters of deionized water. Using adiaphragm compressor, a mixture of TFE (56.3 g/hour) and PMVE (68.6g/hour) is fed at constant rate. The liquid monomer, 8CNVE, is fedseparately at a rate of 3.4 g/hour. The temperature is maintained at 85°C., the pressure at 4.1 MPa (600 psi), and the pH at 6.3 throughout thereaction. Emulsion copolymer solids ultimately reach 26 wt %. Polymeremulsion is removed continuously by means of a letdown valve and theunreacted monomers were vented. The fluorocarbon elastomer copolymer isisolated from the emulsion by first diluting it with deionized water atthe rate of 8 liter deionized water per liter of emulsion, followed byaddition of 640 cubic centimeters of a magnesium sulfate solution (100 gmagnesium sulfate heptahydrate per liter of deionized water) per literof emulsion at a temperature of 60° C. The resulting slurry is filtered,and the polymer solids obtained from a liter of emulsion arere-dispersed in 8 liters of deionized water at 60° C. After filtering,the wet crumb is dried in a forced air oven for 48 hours at 70° C. Theyield of fluoroelastomer copolymer is 82.5 g per hour of reactoroperation. The copolymer composition is 46.7 wt. % PMVE, 3.04 wt. %8CNVE, the remainder being TFE. The copolymer has an inherent viscosityof 0.91 measured in a solution of 0.1 g copolymer in 100 g of Flutec®PP-11 (F2 Chemicals Ltd., Preston, UK).

Comparative Example 11

The general process described in the above example to produce afluorocarbon elastomer copolymer of TFE, PMVE and 8CNVE is repeatedexcept that DA was used alone without PFPEA 2.

Three aqueous streams are each fed continuously to a 1 litermechanically stirred, water jacketed, stainless steel autoclave at arate of 81 cubic centimeters/hour. The first stream consists of 35.4 gammonium persulfate in 3 liters of deionized water. The second consistsof 240 g of the DA in 4 liter of deionized water. The third streamconsists of 29.8 g of sodium hydroxide and 29.3 g sodium sulfite in 3liters of deionized water. Using a diaphragm compressor, a mixture ofTFE (61.7 g/hour) and PMVE (51.8 g/hour) was fed at constant rate. Theliquid monomer, 8CNVE, was fed separately at a rate of 3.1 g/hour. Thetemperature is maintained at 75° C., the pressure at 4.1 MPa (600 psi),and the pH at 8.9 throughout the reaction. The copolymer emulsion isremoved continuously by means of a letdown valve and the unreactedmonomers were vented. Overall, the polymerization runs poorly. Maximumemulsion copolymer solids only reaches approximately 2 wt %. Theemulsion produced is unstable, and most of the fluorocarbon elastomercopolymer that is produced coagulates and deposits in the reactor. Thesmall amount of copolymer recovered is hard and brittle and is notanalyzable by standard methods.

Examples 16-19, Comparative Examples 12-14

The process of the invention is illustrated in the polymerization ofmelt-processible copolymers of tetrafluoroethylene (TFE) withperfluoro(alkyl vinyl ether), i.e., perfluoro(propyl vinyl ether)(PPVE). Three short chain fluorosurfactants as indicated in Table 6A areemployed in a polymerization agent combination with PFPEA 2 and comparedto the short chain fluorosurfactants used alone as a polymerizationsurfactant. Table 6B summarizes the results.

TABLE 6A TFE/PPVE Polymerization Total Surfactant PFPEA APS EthaneMonomer Example g ppm g ppm g g g Ex. 16 C₃F₇OCF₂CF₂COONH₄ 4.01 568 1142 0.516 0.5 2511 Comp. 12 C₃F₇OCF₂CF₂COONH₄ 5 709 0 0 0.549 0.5 2519Ex. 17 C₂F₅OCF(CF₃)COONH₄ 15 2064 3.75 573 0.620 0.5 1818 Ex. 18C₂F₅OCF(CF₃)COONH₄ 12 1701 3 425 0.589 0.5 1834 Comp. 13C₂F₅OCF(CF₃)COONH₄ 15 2127 0 0 0.455 0.5 1752 Ex. 19 C₄F₉CH₂CH₂SO₃H 13.61914 3.4 478 0.607 0.5 3651 Comp. 14 C₄F₉CH₂CH₂SO₃H 17 2378 0 0 0.7510.5 3693

TABLE 6B TFE/PPVE Polymerization Completion Undispersed Kick-off TimeSolids M. Pt. PPVE Polymer Example (min.) (min.) wt % (° C.) wt % MFR(g) wt % Ex. 16 2 74 24.2 310.1 3.72 9.9 0.81 0.04 Comp. 12 1 96 20.6311.9 3.58 7.0 177 8.9 Ex. 17 4 95 12.8 307.3 4.7 35.4 0 0 Ex. 18 4 8315.0 308.2 4.58 38.9 0 0 Comp. 13 2 70 12.2 311.6 3.28 4.2 80 7.7 Ex. 192 117 30.0 309 4.34 29.7 36 1.2 Comp. 14 3 134 25.5 309.8 4.23 4.24 90028

Example 16

Polymerization agent for this example is prepared as a concentrate byadding 4.01 g of C₃F₇OCF₂CF₂COONH₄ to 16.04 g of deionized water in avial and shaking the vial to until the mixture is clear. PFPEA 2 (1 g)is added and then 6 drops of 30 wt % ammonium hydroxide. The vial isagain shaken to give a clear to slightly hazy mixture. This mixture isadded to 900 ml of deionized water with agitation to give a clear (hazeless than about 3%) polymerization agent concentrate of pH 8.

Following the general procedure of Examples 1-3, 900 ml the abovepolymerization agent concentrate is added dropwise with agitation to6100 g of deaerated deionized water in a 3 gallon (11 liter) horizontalautoclave. Ethane, 0.5 g, and perfluoro(propyl vinyl ether) (PPVE), 68ml (104 g) and are added. The autoclave is heated to 75° C. andagitation is begun (70 rpm). The autoclave is pressured to 300 psig(2.17 MPa) with tetrafluoroethylene (TFE). Ammonium persulfate (APS)initiator (48 ml of a 6.2 g/1000 ml water solution) is added initiallyand then 0.54 ml/min of APS initiator solution (6.2 g/1000 ml) is addeduntil the end of polymerization. Table 6A shows the concentration ofpolymerization agent components in the water at the start ofpolymerization. Polymerization is recorded as beginning when TFEpressure drops 10 psi (70 kPa), this point being identified as kick-off.After kick-off TFE is fed to maintain the autoclave pressure at 300 psig(2.17 MPa), and PPVE is fed at the rate of 42 ml/hour. After 10 minutesPPVE feed is increased to 55 ml/hour. When the total monomers, includingthe monomer added before kick-off, stated in Table 6A is added,polymerization is stopped and the autoclave is vented. Polymerdispersion is drained from the autoclave, weighed, its percent solids ismeasured, then it is filtered to remove coagulated (undispersed) solids,and then frozen and thawed to coagulate the dispersed polymer. Thecoagulate is dried overnight in a vacuum over at 100-110° C. at 6-10 mmHg (0.8-1.3 kPa). Melt flow rate, melting point, and PPVE content aremeasured. Results are summarized in Table 6B.

Comparative Example 12

This Example is like Example 16 except that a polymerization surfactantsolution is made by adding 6.0 g of C₃F₇OCF₂CF₂OOONH₄ to 900 ml ofdeionized water with mixing. The resulting mixture is clear with a pH of7. No PFPEA 2 is added. Except that the PPVE feed is not increasedduring polymerization, polymerization and polymer isolation is asdescribed in Example 16. Results are summarized in Tables 6A and 6B. Incomparison to Example 16, which has very little undispersed polymer,8.9% of the polymer made in this Example is undispersed, showing thatC₃F₇OCF₂CF₂OOONH₄ alone is a poor surfactant, though it performs well incombination with PFPEA 2.

Example 17

Polymerization agent for this Example is prepared by:

a) adding 1.3 g PFPEA 2, 1.75 g t-butanol, 2.06 g deionized water, and0.2 g 30 wt % ammonium hydroxide in a vial. After shaking, the mixtureis clear. The mixture is added dropwise to 510 g deionized water withmixing. The resulting mixture, Mixture A, is clear, with a pH of 9.

b) C₂F₅OCF(CF₃)COOH, 5.2 g is placed in a vial and 3.0 g of 30 wt %ammonium hydroxide is added. The capped vial is shaken under runningcold water to remove the heat of neutralization. The resulting mixtureis clear. It is added to Mixture A. The resulting mixture, Mixture B,remains clear. It has a pH of 10.

c) In order, PFPEA 2, 2.45 g, 3.3 g t-butanol, 3.88 g deionized water,and 0.47 g 30 wt % ammonium hydroxide are added to a vial, which iscapped and shaken until the contents are a single phase and clear. Thismixture is added dropwise with vigorous agitation to 500 ml of deionizedwater. The resulting mixture is clear, with a pH of 10. This is MixtureC.

d) C₂F₅OCF(CF₃)COOH, 9.8 g, is added to a vial, after which 3.53 g of 30wt % ammonium hydroxide is also added. The capped vial is shaken underrunning cold water to remove the heat of neutralization. The resultingmixture is clear. This mixture is added dropwise with vigorous mixing toMixture C, giving Mixture D, which is clear and has a pH of 9.5.

Mixtures B and D are then mixed together to produce Mixture BD which isclear (haze less than about 3%).

The polymerization procedure of Example 16 is followed using Mixture BDas the polymerization agent concentrate. The results are summarized inTables 6A and 6B.

Example 18

Polymerization agent for this Example is prepared by adding to a vial inorder: 3.0 grams of Krytox® FSL, 4.0 grams of t-butanol and 4.75 gramsof DI water. This is capped and shaken vigorously to produce a milkymixture. 0.5 grams of 30 wt % ammonium hydroxide is added to the vialand again the vial was capped and shaken to produce a foamy mixturewhich upon standing became a water clear single phase. The contents ofthis vial is added slowly to 900 grams of deionized, deaerated waterwhile stirring vigorously. The resulting mixture is water clear.

To a second vial is added 12 grams of C₂F₅OCF(CF₃)COOH. To this is added4.3 grams of 30 wt % ammonium hydroxide. The vial is capped and shakenunder running cold water to remove the heat of reaction. The resultingmixture is water clear. The contents of this second vial are also addedslowly to the above Krytox mixture while agitating vigorously. Theresulting polymerization agent concentrate is water clear (haze lessthan about 3%) with a pH 9.5 as measured by pH paper.

The polymerization procedure of Example 16 is followed. The results aresummarized in Tables 6A and 6B.

Comparative Example 13

Polymerization agent for this Example is prepared by adding 15 g ofC₂F₅OCF(CF₃)COOH and 5 g of 30 wt % ammonium hydroxide. The vial iscapped and shaken under running cold water to remove the heat ofreaction. The resulting mixture is water clear. It is added dropwisewith stirring to 900 ml of deionized water, producing a slightly hazysolution of pH 8.5. Nine drops of 30 wt % ammonium hydroxide brings thepH to 9.5. The mixture is still slightly hazy.

The polymerization procedure of Example 16 is followed. The results aresummarized in Tables 6A and 6B. In comparison to Examples 17 and 18,which differ in having PFPEA 2 included in the polymerization agent,this Comparative Example 13 makes dispersion containing 7.7% undispersedpolymer. Examples 17 and 18 produced no undispersed polymer.

Example 19

Polymerization agent is prepared by adding 13.6 grams of C₄F₉CH₂CH₂SO₃Na(FORAFAC™ 42) to 900 gm of deionized, deaerated water. This is stirredvia magnetic stir plate until the solid is completely dissolved. Theresulting mixture is water clear with pH=9.5.

In a vial is added 3.4 grams of PFPEA 2, 4.53 grams t-butanol and 5.41grams of deionized water. The vial is capped and shaken to produce amilky dispersion. To this is added 0.58 grams of a 30 wt % ammoniumhydroxide. The vial is again capped and vigorously shaken to produce afoamy mixture. Upon standing, the contents form a water clear singlephase. The vial contents are added to the above 913.6 gram mixture ofC₄F₉CH₂CH₂SO₃Na through a pipette which is immersed under the surface ofthe clear liquid and into the high shear region created by vigorouslystirring the mixture on a magnetic stir plate. Once all of the vialcontents are added, the polymerization agent concentrate is water clear(haze less than about 3%) with a pH=10.5 as measured with pH paper.

The polymerization procedure of Example 16 is followed except that 6200g of deaerated water is used. The results are summarized in Tables 6Aand 6B.

Comparative Example 14

Polymerization surfactant solution for this Example is prepared byadding 17 grams of C₄F₉CH₂CH₂SO₃Na (FORAFAC™ 42) to 900 gm of deionized,deaerated water. This is stirred via magnetic stir plate until the solidis completely dissolved. The resulting mixture is water clear withpH=9.5.

The polymerization procedure of Example 16 is followed except that 6200g of deaerated water is used. The results are summarized in Tables 6Aand 6B. In comparison to Example 19, in which C₄F₉CH₂CH₂SO₃Na iscombined with PFPEA 2 as a polymerization agent in the polymerization,which gives only about 1% undispersed polymer, C₄F₉CH₂CH₂SO₃Na alone aspolymerization surfactant gives a polymer dispersion containing 30%undispersed polymer.

What is claimed is:
 1. A process comprising polymerizing at least onefluorinated monomer in an aqueous medium containing initiator andpolymerization agent to form an aqueous dispersion of particles offluoropolymer, said polymerization agent comprising: fluoropolyetheracid or salt thereof having a number average molecular weight of atleast about 800 g/mol; and fluorosurfactant having the formula:[R¹—O_(n)-L-A⁻]Y⁺ wherein: R¹ is a linear or branched partially or fullyfluorinated aliphatic group which may contain ether linkages; n is 1; Lis a linear or branched alkylene group which may be nonfluorinated,partially fluorinated or fully fluorinated and which may contain etherlinkages; A⁻ is a carboxylate; and Y⁺ is hydrogen, ammonium, or alkalimetal cation; with the proviso that the chain length of R¹—O_(n)-L- isnot greater than 6 atoms, wherein said polymerization agent comprises aminor amount of said fluoropolyether acid or salt thereof and a majoramount of said 25 fluorosurfactant; and wherein said polymerizingproduces less than about 3 wt % undispersed fluoropolymer based on thetotal weight of fluoropolymer produced.
 2. The process of claim 1wherein said chain length of R¹—O_(n)-L- is 3 to 6 atoms.
 3. The processof claim 1 wherein said chain length of R¹—O_(n)-L- is 4 to 6 atoms. 4.The process of claim 1 wherein said chain length of R¹—O_(n)-L- is 3 to5 atoms.
 5. The process of claim 1 wherein said chain length ofR¹—O_(n)-L- is 4 to 5 atoms.
 6. The process of claim 1 wherein when saidpolymerization agent comprises a fluorosurfactant to fluoropolyetherweight ratio of 5:1, said polymerization agent has a surface tension inwater at a concentration of 6000 ppm at 23° C. of at least about 30%less than the surface tension of the fluorosurfactant alone in water at23° C. at a concentration of 6000 ppm.
 7. The process of claim 1wherein: R¹ is a linear or branched partially or fully fluorinated alkylgroup having 1 to 3 carbon atoms which may contain ether linkages; and Lis an alkylene group selected from —CX(R²)—, wherein R² is fluorine orperfluoromethyl and X is hydrogen or fluorine, and —CZ¹Z²CZ³Z⁴—, whereinZ¹, Z², Z³, and Z⁴ are independently selected from the group consistingof hydrogen and fluorine.
 8. The process of claim 7 wherein: L is analkylene group selected from the group consisting of —CF(CF₃)—, —CF₂—,—CF₂CF₂—, —CHFCF₂—, and —CF₂CHF—.
 9. The process of claim 1 wherein R¹is a linear partially or fully fluorinated alkyl group having 2 to 3carbon atoms.
 10. The process of claim 1 wherein R¹ is fullyfluorinated.
 11. The process of claim 1 wherein: R¹ is CF₃CF₂CF₂—; L is—CF(CF₃)—; and Y⁺ is hydrogen or ammonium.
 12. The process of claim 1wherein said aqueous medium contains less than about 300 ppm ofperfluoroalkane carboxylic acid or salt fluorosurfactant having 8 ormore carbon atoms based on the weight of water in said aqueous medium.13. The process of claim 1 wherein said fluoropolyether acid or saltthereof having a number average molecular weight of at least about 800g/mol comprises acid groups selected from carboxylic acid, sulfonicacid, sulfonamide, phosphonic acid.
 14. The process of claim 1 whereinsaid fluoropolyether acid or salt thereof has a number average molecularweight of about 800 to about 3500 g/mol.
 15. The process of claim 1wherein said fluoropolyether acid or salt thereof has a number averagemolecular weight of about 1000 to about 2500 g/mol.
 16. The process ofclaim 1 wherein said polymerization agent is present in said aqueousmedium in an amount of about 5 ppm to about 10000 ppm based on theweight of water in said aqueous medium.
 17. The process of claim 1wherein said polymerization agent is present in said aqueous medium inan amount of about 5 ppm to about 3000 ppm based on the weight of waterin said aqueous medium.
 18. The process of claim 1 wherein said aqueousdispersion of particles of fluoropolymer formed by said process has afluoropolymer solids content of at least about 10 wt %.
 19. The processof claim 1 wherein said aqueous dispersion of particles of fluoropolymerformed by said process has a fluoropolymer solids content of about 20 wt% to about 65 wt %.
 20. The process of claim 1 wherein said at least onefluorinated monomer is selected from the group consisting oftetrafluoroethylene (TFE), hexafluoropropylene (HFP),chlorotrifluoroethylene (CTFE), trifluoroethylene,hexafluoroisobutylene, perfluoroalkyl ethylenes, fluorovinyl ethers,vinyl fluoride (VF), vinylidene fluoride (VF2),perfluoro-2,2-dimethyl-1,3-dioxole (PDD),perfluoro-2-methylene-4-methyl-1,3-dioxolane (PMD), perfluoro(allylvinyl ether) and perfluoro(butenyl vinyl ether).
 21. The process ofclaim 1 wherein said particles of fluoropolymer produced comprise PTFEor modified PTFE having a comonomer content of no greater than about 1wt %.
 22. The process of claim 23 wherein said PTFE or modified PTFE hasa melt creep viscosity of at least about 10⁸ Pa·s.
 23. The process ofclaim 1 wherein said fluorosurfactant comprises at least about 55 wt %of said polymerization agent.
 24. The process of claim 1 wherein saidfluorosurfactant comprises at least about 65 wt % of said polymerizationagent.
 25. The process of claim 1 wherein said particles offluoropolymer produced comprise a melt-processible copolymer comprisingat least about 60-98 wt % tetrafluoroethylene units and about 2-40 wt %of at least one other monomer.
 26. The process of claim 1 wherein saidaqueous medium is substantially free of perfluoropolyether oil.
 27. Theprocess of claim 1 wherein said polymerization medium is substantiallyfree of fluoropolymer seed at polymerization kick-off.